The invention relates to a television synchronization signal processing circuit, which is used when a video signal is read out from disk memory in a still video floppy system and displayed on a television screen.
In the standard television system, one-half interlace scanning is adopted. For example, in the NTSC system, one field is 262.5 H (H being the period of horizontal synchronization signal), and one frame consists of 2 fields, i.e., 525 H. When a reproducing signal from a video disk of the still video floppy system where the video data of one field, i.e., 262.5 H is recorded for each rotation of the disk, there is a deviation of 0.5 H in the horizontal sync pulse, i.e., one half the period of the horizontal synchronization signal, at the juncture, i.e., start or end point, of the recording track signal. This deviation causes distortion in image reproduction with an ordinary television receiver. Accordingly, it is necessary to delay the reproduced signal read out from the disk for every other field by 0.5 H from the juncture of the record to obtain a continuous reproduced horizontal sync signal.
The vertical synchronization signal, on the other hand, is not delayed at all. In this way, the standard one-half interlace scanning is performed. In this case, still picture reproduction has the following drawback. When one horizontal scanning line representing a white line is generated between two horizontal scanning lines each representing a black line, or when a horizontal scanning line is generated between two horizontal scanning lines respectively representing black and white lines, the displayed raster will vertically vibrate for every field by the distance between two adjacent horizontal scanning lines. That is, the raster of one field where the horizontal synchronization signal is delayed by 0.5 H is moved upward or downward with respect to the raster of the other field if the horizontal synchronization signal is not delayed.
To eliminate this drawback, it is necessary to control the phase of the vertical synchronization signal such that the rasters of all the fields will assume substantially the same position.